Which atom has the largest atomic radius?
(1) potassium (3) francium
(2) rubidium (4) cesium

Answer is: 3) francium.

The atomic radius of a chemical element is a measure of the size of its atom.

The atomic radius varies with increasing atomic number, but usually increases because of increasing of number of electrons.

Potassium (K), rubidium (Rb), ceasium (Cs) and francium (Fr) are in group 1 (the alkali metals) of the periodic tabl.

The atomic radius increases down the group of Periodic table and francium is on the bottom of the group 1 with highest atomic number.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.